Recently, for example, a liquid crystal display device has been used widely in a liquid crystal television, a monitor, a mobile phone, etc., as a flat panel display that has features such as a smaller thickness and a lighter weight as compared with a conventional Braun tube. A known liquid crystal display device of this kind uses, in a liquid crystal panel as a display panel, an active matrix substrate in which a plurality of data lines (source lines) and a plurality of scanning lines (gate lines) are arranged in a matrix, and pixels, each having a TFT (Thin Film Transistor) as a switching element and a pixel electrode connected to the thin film transistor, are arranged in a matrix in the vicinities of intersections of the data lines and the scanning lines.
In the active matrix substrate as described above, the pixel electrode and a drain electrode of the thin film transistor are connected to each other via a contact hole portion. Specifically, in the active matrix substrate, generally, a protective layer is formed on the thin film transistor, and an (interlayer) insulating layer is formed on the protective layer. Further, in the contact hole portion of the active matrix substrate, by providing a contact hole in each of the protective layer and the insulating layer, the pixel electrode and the drain electrode are brought into contact with each other, whereby these electrodes are connected to each other.
Further, in the conventional active matrix substrate, generally, the contact hole of the protective layer is formed by etching the protective layer, using the insulating layer formed with the contact hole as a mask. However, in the conventional active matrix substrate, the protective layer sometimes is undercut excessively under the insulating layer during the etching, which makes a profile of the pixel electrode worse, disconnects the pixel electrode due to an improper connection of the pixel electrode to the drain electrode, etc.
Therefore, in the conventional active matrix substrate, as described in Patent Document 1 below, for example, after forming the contact hole of the protective layer by the etching, the insulating layer in the periphery of the contact hole of the protective layer is removed by performing an ashing process to expose a borderline of the protective layer in the contact hole. Thus, in this conventional active matrix substrate, an undercut structure caused in the protective layer is removed. It also is indicated that, in the contact hole portion of this conventional active matrix substrate, by forming a sidewall around the contact hole of the insulating layer and a sidewall around the contact hole of the protective layer in, for example, a stepped configuration, the occurrence of the above problems are suppressed further.